The present invention relates to a belt for a continuously variable transmission.
Such a belt, generally known as a xe2x80x9cpush beltxe2x80x9d and in this particular design alternatively denoted single package push belt, is generally known from a plurality of publications. One early publication of a so-called single package belt is U.S. Pat. No. 3,720,113. The single package push belt is highly desired for practical application compared to a double package belt as known from later publications like EP-A 0000802, but suffers from costly technical measures at providing separate retaining means to keep the carrier and transverse elements of the belt as a whole, in other words to prevent the elements from falling apart from the carrier and to maintain its structural coherence. This maintaining of the integrity of the belt particularly is an issue while the belt is in operation, but is also a consideration at simply maintaining the integrity of a belt assembly.
The prior art shows several ways of solving the above-mentioned problem. In particular, several types of carrier receiving recesses of the transverse element are shown. Amongst these are the provision of one or more axially extending slots, the provision of element parts for hooking the carrier, and the provision of retaining means for locking the carrier receiving recess such as closure pins.
In the belt disclosed by the said US publication, radially extending limb parts of the element are bent axially inwardly with respect to the recess, thereby creating a hook like part, so that the carrier, in addition to it being surrounded along its lateral and its radial inner sides, is at least partly surrounded along its radial outer side too. This construction of hooking of the carrier by the transverse element is favourable for it""s relatively low weight. However, the construction suffers from manufacturing difficulties in that the inward bending of the transverse element provides for a relatively complicated blanking shape and in that at assembly specific effort is required to insert the carrier in the desired position relative to the transverse elements. In particular, the carrier can not be inserted freely, i.e. without deformation thereof. Such deformation may be an initiator of carrier breakage when under severe tensile load during operation. Another construction of such kind is disclosed in EP-A 0095257, where the carrier is relatively tightly hooked into a contacting position with the element.
A construction, which overcomes the latter insertion problem, is known from EP-A 0135237. In this known construction an element shows a so-called slot, being a laterally oriented recess in the element having an opening towards an upper corner of the element. At least part of the radial outer side of the carrier is contacted by a radial outer, or upper element part, and the elements are to be inserted in an alternating sequence. Belts comprising such elements come with a relatively high weight, thus limiting the performance in terms of transmittable torque. Also the alternating sequence requirement brings along a manufacturing complication in that the elements have to be assembled into the belt alternatingly positioned, i.e. mirrored about the longitudinal direction of the belt.
An alike earlier construction showing a favourably more simple slot construction is known from EP-A 0073962. This document discloses elements with a more or less straight slot having an opening towards an axial side of the element. For preventing the risk of the carrier coming into contact with a pulley face while the belt is in operation, also with this construction the elements have to be fitted mirrored. The complication of mirrored adjacent elements is suggested to be overcome by producing the lower, tapered element part with a two sided tapering so that, expectedly at the expense of some production difficulty and cost, only one element orientation need be applied.
Another known single package design shows a separate element type for maintaining belt integrity at operation of the belt. Such element type is provided with a retaining means in the form of locking element, e.g. a pin or a locking ring, closing of the carrier receiving recess after assembly thereof. Such a construction may render a secured integrity of the belt, both in operation and as a separate part, but is relatively unfavourable due to weight and manufacturing cost, while the robustness in many designs can not be guaranteed when somewhat larger torque levels are to be transmitted, such as e.g. typically required for automotive application. Of this belt design, one such pin construction is disclosed in EP-A 0122064, and one such ring construction is disclosed in U.S. Pat. No. 5,123,880.
The present invention principally, but not exclusively, aims at realising a single package belt design without above mentioned cost raising disadvantages, which is composed of only a single element type, which has a relatively uncomplicated shape and a relatively low weight so as to maximise the amount of torque transmittable by the belt, while still responding in a good manner to the various functional demands for a belt. Such a belt is according to the invention is discussed below.
A belt of the invention may be produced easily since in principle no carrier retaining means is required during operation. Surprisingly, in the design according to the invention the function of the retaining means of maintaining belt integrity may be achieved by favourably utilising and adapting some geometric features inherently present within a belt. Here setting a specific ratio between a circumference length of the belt, i.e. its longitudinal dimension when measured at the radially inner side of the carrier, and the height of the upper element part of the transverse elements.
The solution is based on the insight that at a slack in between pulleys part of the belt""s trajectory when applied in a transmission, which is considered the most critical part in view of disintegration of the belt by the invention, the elements may block one another in a stable end position in which the relevant belt part is concavely shaped. The solution takes account of the experience and insight that such slack trajectory part has a tendency to buckle radially inward as a result of a clamping action on the belt by the pulleys, which tends to counteract the belt exiting the pulleys and urging the belt to continue to rotate in the pulley.
According to the invention the current design feature may favourably be applied to the double package belt designs. For the double package design this feature enables a different design of the element with respect to the known art. In particular the transverse width of the T-shaped upper element part may be reduced, since it no longer serves to limit the radial play between carrier and element. Moreover, the feature aids in securing or optimising the belt""s functional lifetime, because the carrier is loaded less during operation of the belt. There e.g. no longer needs to be an interaction between the radial outer side of carrier and the element, which in the belt according to the known art occurs at the location where the elements exit a pulley. In this area the, due to elastic deformation of the pulley, the elements are clamped between the sheaves of the pulley very effectively such that they tend to keep rotating along with the pulley. In the known belt, the elements are pulled away from the pulley sheaves by the carrier interacting with the retaining means, such as the T-shaped upper element part or the locking pin, putting an additional load on those components. In the belt according to the invention, the elements of the belt are kept firmly in contact also in the slack trajectory part, so that a force for pulling the elements away from the pulley may be transmitted from one element to the other.
Alternatively, with the solution according to the invention favourably large values of a total end play, being the summation of the longitudinal play between the elements of the belt, may be allowed thus favouring the assembly of the belt, provided the height of the element above the rocking edge is made sufficiently large. The invention hereby among others may take advantage of the insight according to which only a part of a belt""s total end play is present in the slack trajectory part at any time. Moreover, the end play in this slack trajectory part will generally not be concentrated between two adjacent elements, but tends to be more or less distributed. Accordingly, the element height need only be dimensioned, for the largest gap, i.e. play, that may be expected to occur during operation between two adjacent elements, which may for instance be established in practice.
A specific difficulty has been overcome in the invention by taking account of the belts total end play, as e.g. may be imagined by having the belt elements stacked along the carrier in a mutually contacting manner. In a further elaboration of this principal solution, the relation departs from the square root of the belt""s circumference length, being defined as the length of the radially inner periphery of the carrier. A special advantage of the solution according to the invention is that it not only takes account of the actual end play of the belt when in free state, i.e. circularly shaped, such as is possible outside the transmission, but that it also covers the situation where the total end play is partly determined by elastic deformation of carrier and elements during operation or becomes larger still due to its mounting and operation within a transmission.
The described problem of maintaining the integrity of the package belt is according to the invention alternatively solved by a feature which is here incorporated in addition to the preceding solution, but which may also be applied exclusively. This feature entails that at least the majority of the transverse elements of the belt have a thickening of the upper element part relative to the thickness of the element at the location of its rocking edge, i.e. the largest thickness of the lower element part. The upper element part thus also, albeit slightly, being tapered. In this way a so called xe2x80x9ccat""s back-like arcxe2x80x9d is achieved in an array of elements being pushed against each other as arranged between two pulleys. Exiting of the belt from the pulley is enhanced, while the elements do not fall apart from each other or from the carrier, since they are wedged in their position when observed from side elevation. With the addition, or presence as the case may be, of this feature, the belt may, at standstill of a transmission, be incorporated without the presence of a spring in a pulley that usually is adopted to provide a basic clamping force in all transmission conditions. This is because such wedge shaped elements, at least when located at a transmissions upper side with respect to gravity, prevent the local belt part from assuming a particular shape from which the belt may no longer transform to a desired rod like condition during operation. In the slack trajectory part of the belt, whether at the transmissions upper or lower side, the wedging feature aids in reducing the play between the elements and thereby in assuming a stable concave belt shape of that belt part.
Surprisingly, the measure according to this latter aspect of invention again aids in maintaining the integrity, or coherence, of the belt while obviating the application of the previously described known retaining means by positively or directly limiting the relative radial movement between transverse element and carrier. This enables the element design wherein the recess is entirely open in radial outward direction, at least without hook parts or locking means being provided capable of retaining the carrier. For achieving this effect, according to the invention only a small but noticeable amount of thickness increase of the upper element part over its radial dimension is required. With this feature, advantage is taken of the circumstance that a belt incorporates a significantly large number of transverse elements.
The invention thus also relates to the belt, wherein at least the thickness of the upper element part of a majority of the transverse elements in the longitudinal direction of the belt is notionally larger than a largest thickness of the tapered lower element part, such that an array of elements pushed against one another within the belt assumes an arc-like shape. In a favourable embodiment of the invention, the thickness of the upper element part, i.e. extending radially outward from the rocking edge gradually increases. In this way the wedging effect is achieved over a considerable if not maximum distance on the element thus, optimising the security of the wedging effect.
It is here remarked that in the context of the current description the height of the element is assumed to concur with the dimension of the belt in the radial direction when positioned in a circular shape, the width of an element concurs with the dimension of the belt in the transverse or axial direction and the thickness of the element concurs to its dimension in the belt""s longitudinal direction, i.e. along the circumference of the belt.
More favourably, the tapering of the upper element part is achieved with one longitudinally facing side of the element forming a flat plane covering both a substantial part of the lower element part and of the upper element part. In this way the problem of technical and economical manufacturing of the element is favourably solved, e.g. only one longitudinally facing face of element is profiled favourably allowing element production by means of blanking.
A particularly favourable and secure effect may according to the invention be achieved when a nominal difference in thickness of the belt""s elements between the upper element part and the lower element part at the level of the rocking edge is between 0.01 and 0.20, preferably about 0.04 times the thickness at the rocking edge.
More in particular, the invention relates to a so called single package belt in which the upper element part is provided with limbs extending in a direction to the radial outside from the rocking edge, each limb being provided near a lateral side portion of the element adjoining an axial end of the carrier contacting face, alternatively denoted saddle surface or simply saddle, and defining a lateral boundary of the recess. Unlike the known belt designs, this type of belt may readily be produced with a variety of carriers dimensions for covering a variety of applications e.g. differing in torque transmitting capacity. The element type described may be kept relatively low weight, because of a lack of material over, a large part of, the width of the carrier in the upper element part.
According to another specific aspect of the invention, each such limb is provided with a predominantly longitudinally oriented protrusion, here denoted protrusion, at a level above the radial outer side of the carrier when in contacted with the element through the saddle surface. In this structure, the protrusion promotes the maintaining of the integrity of the belt particularly at the slack trajectory part, while allowing a relatively large total end play. When the belt at such slack trajectory part is bent towards the centre of the belt, i.e. concavely bent, the elements tend to mutually contact through radially outer parts thereof. Providing the protrusion close to or even at these outer parts is according to the invention intended to prevent the elements to move relative to each other in the radial direction, and therefor prevents the single package belt from disintegrating. This function of the protrusion is not found in known belt arrangements, where it instead has a function at the in between pulleys trajectory part of preventing elements from vibrating axially and at arc-shaped pulley trajectory parts of the belt of supporting a stable mutual orientation of the elements, and, accordingly, requires a different positioning of the protrusion in accordance with the invention. Since this new function and new positioning of the protrusion in it self enables, at least enhances, an adequate functioning of the single package belt design, the invention also relates to a belt in which a limb is provided with a predominantly longitudinally oriented protrusion at a level radially outward from the carrier when in contacted with the element through the saddle surface.
In such new arrangement, preferably the protrusion is located entirely in an upper half of the radial dimension of the limbs. In a favourable embodiment the protrusion has a predominantly axially extending lower surface forming a radially inner boundary of the protrusion, thereby enhancing both the protrusion""s function of preventing radial displacement between adjacent elements at an in between pulleys trajectory part, as well as the manufacturability of the element. As in the known art, with the protrusion arrangement according to the invention being located on a longitudinally facing surface of the limb, a corresponding hole is provided in a surface of the limb facing in an opposite direction. For optimally securing the functionality of the new protrusion arrangement, the invention prefers an arrangement wherein the protrusion is produced partly as a part of the limb that extends in a longitudinally forward and a radially outward direction. In this manner the protrusion is realised while maintaining the elements thickness over the entire radial length of the upper element part, thereby enabling the elements to take up a considerable pushing force in a straight part of the belts trajectory. In this new design the protrusion is preferably produced by bending the limb, rather than by shearing of material to simultaneously form the protrusion and the hole as is the preferred method for forming known protrusion embodiments. Yet a further favourable aspect of this new protrusion design is that it may be partly formed by a limb part extending both in radial and in longitudinal direction adjoining a limb part located to the radial outside therefrom and extending in radial direction parallel to a limb part radially below the limb part. Such structure may favourably be produced while maintaining the new functionality of the protrusion part.
Yet a further specific aspect of the single package design according to the current invention shows the limbs having end parts in a radially outward direction to comprise a section of axial limb width that is increased relative to a limb section near the level of the saddle surface. In this manner a good, i.e. stable and reliable mutual contact of elements, particularly in the slack trajectory part is promoted, thereby enhancing the application of the belt type without the retaining means or, more specifically, of the belt type wherein the transverse width carrier receiving recess of the elements would allow movement into or out of the recess of the carrier without deformation thereof. In a preferred embodiment of this aspect of the invention the section of increased limb width is located at least for a major part thereof radially outward from the protrusion, more preferably directly adjoining or including a top side or radially facing an end face of the limb, thereby minimising the chance of failure in mutual contacting of the limbs, particularly since in accordance with the invention the limbs are otherwise sought to be of minimal width.
In an embodiment favourably to be produced, the fore mentioned section of increased limb width is formed through an axial side of the limb extending according to a straight line as viewed in the longitudinal direction of the belt, preferably extending substantially radially. In such embodiment the top side of the limb preferably may be provided with a flat surface extending predominantly axially and longitudinally, which surface merges with axial sides of the limb by a rounded transition section, preferably of a radius of approximately 1 mm.
So as to optimise the good functioning of a single package belt in which the carrier receiving recess would allow free mutual movement of carrier and element, the limbs in longitudinal view extend radially and axially under an angle at least substantially conforming to an angle at which both pulley contact faces of the element and element contact faces of the pulley are oriented, the so called flank angle, at least for a predominant part of the height of the limbs. By this measure the limbs in fact are produced to be axially separate, preferably as much as possible within the confinement of a V-shaped groove defined by the pulley, thereby promoting the stability of the belt in respect of torsion and axial buckling thereof. To which phenomena the belt, especially of the invented kind may be subject to at parts of the belt trajectory in between the pulleys. In a preferred embodiment of this latter element design provided with the protrusion according to the invention, a predominant part of an axially outward facing surface of the protrusion is also oriented at an angle at least substantially conforming to the flank angle. Moreover, a predominant part of an axially inward facing surface, i.e. a surface facing the same surface of the protrusion provided on an opposite limb, may also be oriented at an angle at least substantially conforming to the flank angle, to minimise the dimensions and mass of the protrusion, while optimising element stability.
In accordance with the preceding features of the invention, a favoured embodiment of the element is found in that a mutual separation of the axially inward facing surface of the limbs facing the carrier receiving recess is larger than an axial width of the carrier.
While securing the functionality of a single package belt, the invention seeks to reduce its weight, thereby optimising a torque transferring capability of the belt. Thus irrespective of any of the measures and features described in the above, the invention seeks to keep a nominal axial width of a limb small in relation to the width of the carrier. In this respect, according to the invention the sum of the axial widths of the limbs at a radial level corresponding to that of the saddle surface is less than one third, preferably less than one sixth, of an axial width of such surface, while the recess for receiving the carrier is directed radially outward and opens to the radial outside over a width that at least conforms to the axial width of the carrier. In this optimised design, the radial length of the limbs is favoured to surpass by at least three times the thickness of the element at the level of the saddle surface when departing from the radial level of the saddle surface. By such measure, there is no need for the limb being provided with a hook shaped end part for obstructing the element from separating from carrier in the radial direction, that is without a simultaneous mutual rotation of element and carrier, or, in case of a double package belt, the upper element being T-shaped such that free insertion of the carrier in the recess is possible even when the carrier is oriented parallel to the axial direction. Thus the limbs, or alternatively the upper element part, can be produced having a relatively small axial width, thereby optimising the ratio between carrier width, i.e. the width determining the torque transferring capability and the total width of the belt. Preferably the limb width substantially conforms to the element thickness at the level of the saddle surface face, the limbs than having a square cross section at the level as seen in a direction along the length of the limb.
In a preferred embodiment, favourable per se and in combination with the respective features according to the invention, the elements show a axially extending rocking edge, alternatively and more accurately denoted a tilting zone because usually it is not a sharp edge but a more or less smoothly curved surface in the form of a radially oriented convex curvature that extends over a substantial part of the width of the element. The rocking edge or tilting zone is preferably located at a distance between 0.4 mm to 0.6 mm radially inward from the saddle surface. In this manner relatively large contacting pressures may be received between elements within the belt, while the rocking edge is located close to the saddle surface to minimise speed differences within the belt between carrier and elements during operation.
A particularly important aspect of the invention relates to a belt design wherein the transverse element at each lateral end thereto is provided with a side surface in the form of a pulley contact face, alternatively denoted flank, for contacting the pulley. The flanks extend in radial direction beyond the upper boundary of the carrier at least when in contact with the saddle surface. In this manner a contact pressure between the element and the pulley may be significantly reduced. Although from theoretical considerations it may be taken that an element may correctly receive the clamping force of a pulley by a flank having a smaller radial height, a large flank is favourable both for a stability of the orientation of the element with respect to the pulley and for maintaining favourable lubricating conditions there between. It is observed that a theoretically allowable contact pressure may not be allowable after all due to disintegration of a lubrication medium as a result of the amount of heat being generated in the frictional contact between element and pulley. In extreme cases this heat may cause local micro welding spots between pulley and element, severely reducing belt service life.
In a preferred embodiment the flank extends for at least one third, however, preferably not more than about half of its radial height above the rocking edge, thereby also improving the elements stability within a pulley, because an effective radial level of application of the friction force between element and pulley is then brought to, or at least close to the radial level of the rocking edge where the pushing force between adjacent elements is transmitted. In this manner, a force moment, of the forces is minimised, whereby the tendency of the elements to tilt about an axial of the belt while in the pulley is minimised. In this respect, the radius of the axially extending curvature that forms the rocking edge is about 12 mm or even larger. Preferably, the radius falls within the range of values defined at a minimum by the radial height of the flank and at a maximum by the nominal element width. This range was found to be particularly suitable in the context of the single package belt design as defined by the features of the present invention.
Yet another particular aspect of the invention relates to a belt design in which the lower side of the endless element is shaped with a radially inner edge that is shaped concave, whereby the inner edge extends in axial direction at least over the width of the saddle surface. Although the feature of a concave inner edge is known per se from a double package belt design, it has never been applied in a commercial belt design. Apart from rendering a novel structure, application thereof in the single package belt design, however, favourably increases flank height while keeping the total volume and weight of the lower element part down. In a preferred embodiment, the inner edge is curved at least virtually in agreement with a radius having a value within the range from 0.5 to 1 times the largest transverse width of the lower element part. This range was found to be particularly suitable in the context of the single package belt design as defined by the features of the present invention.
In a favourable elaboration of the latter aspect of the invention, the lower element part may be provided with an axially extending step in the longitudinal direction, such that at such location the thickness of the element changes stepwise. In this manner the weight of the element may be kept down further. According to the invention the step is provided close to the inner edge of the element, preferably intersecting it in at least one location. This design feature has a positive effect on the accuracy of the blanking process for element manufacture and, moreover, reduces the force to be applied during blanking. According to a preferred embodiment of the invention, radially below the step the element has a thickness in the range from 0.75 to 0.90 times the largest longitudinal thickness of the lower element part. Such range providing the optimum balance between element strength for taking up the clamping force and weight reduction as aimed at by the present invention.
Finally, the belt features a favourable design aspect wherein axial ends of the saddle surface smoothly merge with the limbs through small protrusions, whereby the protrusions are shaped predominantly arc-like. In this manner the transition between limbs and saddle surface is less prone to cracking. Moreover, with such design the flank may extend up to a considerable level above the radial level of the saddle surface, while the clamping force at the upper flank portion may still be favourably guided into the lower element part. In an optimised embodiment in this respect, a virtual continuation of the arc like protrusion aligns with the surface of the limbs facing the carrier receiving recess.
The above proposed embodiments and aspects of the invention, either per se or in combination most favourably operate in combination with a carrier composed of a set of radially nested endless flat metal bands that are relatively thin.